Make egui_glow painter to work on web (#868)

Add WebGL1 and WebGL2 support to glow painter.
Add "glow" feature to egui_web to use the glow painter there.
Make winit an optional part of egui_glow

egui_glow/src/shader_version.rs

@@ -0,0 +1,79 @@
+#![allow(unsafe_code)]
+use crate::misc_util::glow_debug_print;
+use glow::HasContext;
+use std::convert::TryInto;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[allow(dead_code)]
+pub(crate) enum ShaderVersion {
+    Gl120,
+    Gl140,
+    Es100,
+    Es300,
+}
+
+impl ShaderVersion {
+    pub(crate) fn get(gl: &glow::Context) -> Self {
+        let shading_lang = unsafe { gl.get_parameter_string(glow::SHADING_LANGUAGE_VERSION) };
+        glow_debug_print(&shading_lang);
+        Self::parse(&shading_lang)
+    }
+
+    #[inline]
+    pub(crate) fn parse(glsl_ver: &str) -> Self {
+        let start = glsl_ver.find(|c| char::is_ascii_digit(&c)).unwrap();
+        let es = glsl_ver[..start].contains(" ES ");
+        let ver = glsl_ver[start..].splitn(2, ' ').next().unwrap();
+        let [maj, min]: [u8; 2] = ver
+            .splitn(3, '.')
+            .take(2)
+            .map(|x| x.parse().unwrap_or_default())
+            .collect::<Vec<u8>>()
+            .try_into()
+            .unwrap();
+        if es {
+            if maj >= 3 {
+                Self::Es300
+            } else {
+                Self::Es100
+            }
+        } else if maj > 1 || (maj == 1 && min >= 40) {
+            Self::Gl140
+        } else {
+            Self::Gl120
+        }
+    }
+
+    pub(crate) fn version(&self) -> &'static str {
+        match self {
+            Self::Gl120 => "#version 120\n",
+            Self::Gl140 => "#version 140\n",
+            Self::Es100 => "#version 100\n",
+            Self::Es300 => "#version 300 es\n",
+        }
+    }
+    pub(crate) fn is_new_shader_interface(&self) -> &'static str {
+        match self {
+            ShaderVersion::Es300 | ShaderVersion::Gl140 => "#define NEW_SHADER_INTERFACE\n",
+            _ => "",
+        }
+    }
+}
+
+#[test]
+fn test_shader_version() {
+    use ShaderVersion::{Es100, Es300, Gl120, Gl140};
+    for (s, v) in [
+        ("1.2 OpenGL foo bar", Gl120),
+        ("3.0", Gl140),
+        ("0.0", Gl120),
+        ("OpenGL ES GLSL 3.00 (WebGL2)", Es300),
+        ("OpenGL ES GLSL 1.00 (WebGL)", Es100),
+        ("OpenGL ES GLSL ES 1.00 foo bar", Es100),
+        ("WebGL GLSL ES 3.00 foo bar", Es300),
+        ("WebGL GLSL ES 3.00", Es300),
+        ("WebGL GLSL ES 1.0 foo bar", Es100),
+    ] {
+        assert_eq!(ShaderVersion::parse(s), v);
+    }
+}